1. Field of the Invention
This invention relates to a Nd:YAG laser, and more particularly to a laser whose oscillation mode is brought to a single longitudinal mode by an etalon disposed in a resonator.
2. Description of the Related Art
There have been provided various solid state lasers where oscillation of Nd:YAG (neodymium-doped YAG) in the range around 0-9 .mu.m is utilized. As disclosed, for instance, in U.S. Pat. No. 5,278,851 and U.S. Pat. Nos. 5,502,738 and 5,506,860, in such lasers, there has been in wide use a technique that a Fabry-Perot etalon is provided in a resonator to bring the oscillation mode to a single longitudinal mode. In this specification, the word "etalon" generally means the Fabry-Perot etalon.
However in the conventional Nd:YAG lasers, where emission in a single longitudinal mode is obtained by use of such an etalon, conditions of the etalon (e.g., thickness, reflectivity and inclination of the etalon) and the resonator (the separation of longitudinal modes) necessary for obtaining a good single longitudinal mode are not known. Accordingly, there has been a problem that when an etalon which has been successful in a certain resonator is used in another resonator as it is, there can be obtained only unsatisfactory single longitudinal mode.
Further, even if an excellent single longitudinal mode can be obtained by virtue of the etalon, it sometimes results in deterioration in the beam profile and/or the output power. Thus the conditions of the etalon and the resonator which must be met in order to obtain a good single longitudinal mode, a good beam profile and a high output power at one time have not been known.
Our investigation has revealed that when the thickness, reflectivity and inclination of the etalon are increased, the beam profile and the output power deteriorate though the single longitudinal mode can be improved and that when the thickness, reflectivity and inclination of the etalon are reduced, properties of the single longitudinal mode deteriorate though the beam profile and the output power can be improved.
When the length of the resonator is increased (that is, when the separation of longitudinal modes in the resonator is reduced), properties of the single longitudinal mode deteriorate and vice versa. However the beam profile and the output power are little affected by the length of the resonator.